This invention relates to an improved method of forming sheet glass by the float process wherein molten glass is delivered onto the surface of molten metal in a bath and is formed into a continuous sheet or ribbon while the glass is floating and advancing on the molten metal, the method being particularly suited to the production of sheet glass thinner than about one millimeter.
In the well known float process, molten glass is allowed to spread on the surface of a molten metal such as tin to form a glass layer which is gradually cooled while it is floating on the molten metal until it becomes a dimensionally stable, flat sheet or ribbon of glass. Under equilibrium conditions the molten glass layer floating on the molten metal will assume an equilibrium thickness of about 6 to 7 mm.
To produce a sheet glass thinner than equilibrium thickness by the float process, it is usual to stretch an unsolidified region of the glass ribbon on the molten metal along the direction of travel of the glass ribbon by applying a pulling or tractive force to the cooled and solidified region of the glass ribbon from the outside of the outlet end of the molten metal bath. In this case it is natural that a sideward constrictive force acts on the glass ribbon, and accordingly it is usual to oppose to the constrictive force by means of top or edge rolls that are arranged along and above the molten metal surface so as to engage lateral marginal regions of the glass ribbon. Therefore, the marginal regions of the glass ribbon at this stage need to be thick enough to surely engage the aforementioned rolls, but it becomes difficult to meet this requirement if the thickness of the glass ribbon in its flat major region is reduced unlimitedly. For this reason, it is very difficult to industrially produce a sheet glass thinner than about 2 mm by this technique.
In view of the above described difficulty, Japanese patent application Publication No. 54(1979)-31012 proposes to control the flow of molten glass on the molten metal by forming a sort of sluice at a short distance from the inlet end of the molten metal bath and heating the molten glass while it advances through this sluice. More particularly, the sluice is provided by an elongate solid block of a heat-resistant and electrically conducting material, which is fixedly disposed slightly above the surface of the molten metal to extend transversely of the advancing flow of the molten glass such that the molten glass on the molten metal in a region between the inlet end of the molten metal bath and the elongate solid block forms a pool and is forced to pass through the gap between the bottom face of the solid block and the surface of the molten metal to advance to the downstream. The heating of the molten glass is effected by making an electric current to flow through the molten glass between the sluice-forming block and the molten metal to generate Joule's heat. Also in this method a tractive force is imposed on the glass layer on the downstream side of the sluice-forming block. In fact, it is possible to produce sheet glass of very small thickness by this method.
However, we have recognized that when sheet glass thinner than about 1 mm is produced by the method of the quoted Japanese patent specification the obtained sheet glass exhibits distortion in the form of a number of continuous lines along the direction of the travel of glass on the molten metal and/or contains a number of small bubbles in the glass body. In our view, it is quite difficult to obtain sheet glass of commercially satisfactory quality by this method when the thickness of the sheet glass is smaller than about 1 mm. Besides, the electric heating of the molten glass at or in the vicinity of the sluice offers various problems to the glass forming operation and becomes a cause of lowering in productivity.